A wide variety of processes and compositions have been proposed for forming the gasket in container closures, for instance bottle caps. These include plastisols, solutions in organic solvents, aqueous dispersions (including aqueous latices) and mouldable thermoplastic compositions. They have all been proposed for a variety of container closure types.
A material that has been used very widely for forming the gaskets of, for instance, beer bottles is polyvinyl chloride plastisol. This gives good sealing properties and, in most instances, adequate impermeability to the migration through the gasket of gases, for instance odours from outside the container. However there is currently a desire to avoid the use of polyvinyl chloride in gaskets for containers for potable materials and so it would be desirable to avoid the use of polyvinyl chloride and to devise instead an alternative polymeric composition that could give sealing properties as good as or better than those obtain using polyvinyl chloride. Amongst the materials that could be considered are thermoplastic compositions.
An early disclosure of the use of thermoplastic compositions for forming container closures is in GB 1,112,025. This discusses a wide variety of ways of introducing the compositions into the cap and a wide variety of thermoplastic compositions that can be used. Thus it describes, for instance, blends of ethylene vinyl acetate (EVA) and micro crystalline wax, EVA and low density polyethylene (LDPE) having a melt flow index (MFI) of 7, similar blends containing also butyl rubber having Mooney viscosity of 70, a blend of equal amounts of LDPE having MFI 7 with butyl rubber having Mooney 70, blends of different types of EVA, a blend of LDPE with polyisobutylene, a blend of EVA with ethylene propylene copolymer, an ethylene acrylic acid ester copolymer, a blend of this with LDPE, a blend of LDPE with ethylene propylene copolymer, and a blend of LDPE with chloro sulphonated polyethylene.
Although cold moulding methods were also described in this and in GB 1,112,023, exemplified processes comprised forming a sheet of the appropriate blend, cutting it into discs and inserting the discs into crown closures.
Various disclosures of forming gaskets from thermoplastic compositions have appeared from time to time since then, for instance in EP 331,485, and these have listed a wide variety of polymers that can be used. Generally, most of the polymers named above have been listed.
A paramount need is to avoid off-tastes permeating from outside the container, through the gasket and into the edible composition. A particular problem arises with chlorinated phenols and chlorinated anisoles. The chlorinated phenols are often applied initially as fungicides and the chloro-anisoles are often generated as microbial metabolites of the chlorinated phenols. Polyvinyl chloride gaskets, despite having been found to be generally very satisfactory, do allow small amounts of permeation of such materials and this can, over a long period, contribute to an off-taste. It is therefore desirable to avoid prolonged storage of containers such as beer bottles close to a source of chlorinated phenol or chlorinated anisole. It would therefore be desirable to be able both to replace polyvinyl chloride and, if possible, to obtain a gasket that is even less permeable to chlorinated phenols and chlorinated anisoles.
Despite the very broad disclosure of polymers in GB 1,112,025, the thermoplastic compositions that have mainly been used have been those where the thermoplastic polymer is polyethylene, ethylene vinyl acetate polymer, or a blend of these with each other. Unfortunately such compositions are much worse, as barriers to chlorinated phenols and anisoles than the traditional polyvinyl chloride gasketing compositions. Also it has long been reciprocated that the best sealing properties of all can often be obtained using a cork inlay, frequently with an aluminium facing. However such gaskets, although very effective, tend to be rather uneconomic and they are not satisfactory for pressurised containers.